Omnidirectional reflected interrogation rejector

ABSTRACT

This application describes a way in which radar-beacon transponders can be made to reject nearly all of the reflected interrogations that reach them without rejecting direct-path interrogations unless two or more interrogators operating on the same interrogation-repetition frequency interrogate a particular transponder simultaneously. Logic circuits are provided to show how the IFF Mark X (SIF), also known as the Air Traffic Control Radar Beacon System (ATCRBS) or the Secondary Surveillance Radar (SSR), can be modified to make use of the new technique. Classified military systems can use it equally well if suitable modifications are made. A circuit is also provided to suppress sidelobe-interrogation decoding instantaneously so that the transponder dead time, no longer needed when reflected interrogations are rejected, can be eliminated.

Unite States Patent Bishop [54] OMNIDIRECTIONAL REFLECTED INTERROGATION REJECTOR [75] inventor: Walton B. Bishop, Oxon Hill, Md.

[73] Assignee: The United States of America as represented by the Secretary of the Navy [22] Filed: July 31,1970

[21] App]. No.: 60,009

52 us. Cl ..343/6.8 LC, 343/65 R 51 Im. Cl ..G0ls 9/56 58 Field of Search.343/6.5 a, 6.5 LC, 6.8 R, 6.8LC

[56] References Cited UNITED STATES PATENTS 3,176,291 3/1965 Majerus ..343/6.8 LC

[ 1 Feb. 6, 1973 Primary Examiner-T. H. Tubbesing Attorney-R. S. Sciascia, Arthur L. Branning, J. G. Murray and S. Sheinbein {57] ABSTRACT This application describes a way in which radarbeacon transponders can be made to reject nearly all of the reflected interrogations that reach them without rejecting direct-path interrogations unless two or more interrogators operating on the same interrogationrepetition frequency interrogate a particular transponder simultaneously. Logic circuits are provided to show how the [FF Mark X (SIF), also known as the Air Traffic Control Radar Beacon System (ATCRBS) or the Secondary Surveillance Radar (SSR), can be modified to make use of the new technique. Classified military systems can use it equally well if suitable modifications are made. A circuit is also provided to suppress sidelobe-interrogation decoding instantaneously so that the transponder dead time, no longer needed when reflected interrogations are rejected, can be eliminated.

4 Claims, 16 Drawing Figures w M V D50 PULSES 4/4 17 REPLY TRIGGERS TRANSPONDER "'6 O 4/5 B A a/A |E{ ISLS R T 2 3.. 1/64/6 MODE 1 i 1 M 423 i .1 MODE 2 a 43/ 437 i 1/ 424 o 6 -j DELAY j 432 7 4/8 f, MODE am 425 D i DELAY a 433 i 426 0 i 4/9 DELAY n 434 H A MODE 9 427 D SHtFT REGlSTER I DELAY r A 435 4 0 DELAY lg 436 E fi MODE c .f'. 42/ P 0 L REPLY TRIGGER GATES 2o W 3L hon (551785 429 22 ISLS rmeeens 4/2 H 430 4 i E MAIN-BEAM NON-MAIMBEAM REFLECTED- REFLECTED- 25 INTERROGATION INTERROGATION REJECTOR RE LY REJECTOR SlDE LOSE INTERROGATION rmeesns uecooz SUPPRESSOR (Freda) (FIG. 5)

( FIG. 7)

PAIENIEUFEB s 1975 SHEET 1 OF 8 DIRECT REFLECTED II I I INTERROGATIONS RECEIVED REPLY TRIGGERS REJECTED REJECTED I F/G. Z

MEASURED INVENTOR. WALTON B. B/SHOP M AI'TORNEY M OMNIDIRECTIONAL REFLECTED INTERROGATION REJECTOR STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The present invention relates to a suppression network for use in transponder receiving interrogation signals.

An interrogation transmitter system, when utilized with a transponder, transmits a coded signal consisting of a train of timewise spaced pulses. The coded signal generated by the interrogation system is received by the transponder and analyzed. If the pulses are of the correct frequency, amplitude and spacing, an output signal will be produced by the transponder. The interrogation system may, for example, be positioned on the ground while the transponder may be mounted in an aircraft and be utilized for identification purposes or for deriving other information such as altitude or bearing of the aircraft.

Since the transponder is triggered, or caused to transmit a reply, in response to interrogation coded signals, care must be taken to prevent false, or spurious triggering of the transponder. Such undesired triggering of the transponder is usually caused by a coded pulse train signal from a side lobe of the radiating antenna of the interrogation system. Another cause for undesired triggering has been known to occur due to echoes or reflected signals.

Current interrogation friend or foe (IFF) transponders include a suppression network to prevent triggering of the transponder due to reflected signals. The suppression period, usually 75 to I25 microseconds, that follows each reception of an interrogation causes the transponder to reject all interrogations during that period, when actually, only the reflected signals should be rejected.

Since all interrogations on any particular mode are essentially identical, regardless of their source or time of occurrence, it is not possible to code them so that a reflected signal can be recognized as being the same as the one just previously received.

There are two types of reflected interrogations that interfere with the operation of radar beacons. The most common type follows a path that is entirely within the main beam of the interrogator antenna. Such reflected interrogations will continue to reach the transponder until the interrogator antennas main beam has moved off of the target (or the target has moved out of the main beam). Normally, as long as current equipment and interrogation repetition frequencies are used, each time a reflected interrogation reaches a transponder, about thirty to forty more follow. Efforts to reduce interrogation repetition frequencies should eventually reduce this number to something less than twenty, but this will not change the reflected interrogation problem. When main beam reflected interrogations occur, a false target, or ghost, will appear in line with the real target and at a greater range. The range difference between real and false target will be equal to the difference between the lengths of the direct-path and the indirect or reflected path between interrogator and transponder.

Transponders now in use throughout the United States and Europe reduce the effect of reflected interrogations by suppressing all transponder interrogations for about usec immediately after receipt ofa mainbeam interrogation. This suppression serves to deny a response to any interrogation arriving via a reflected path, but it also denies responses to other direct-path interrogations that may arrive from other interrogators during this time interval.

A technique for preventing such reflected lFF interrogations from triggering an aircraft transponder without having this long suppression period is taught and claimed in copending U.S. Pat. application of Walton B. Bishop, Ser. No. 22,467, filed Mar. 25, 1970, now U.S. Pat. No. 3,646,556. That patent application describes a technique for overcoming the effect of [FF interrogation reflections whose entire path is within the main beam of the interrogator antenna. The technique consists of measuring the time between arrival of the direct-path and the first indirect-path interrogations, and then using this information to reject succeeding indirect-path interrogations of the same delay. It thus eliminates the need for the long (125 asec) transponder decoder suppression periods that follow reception of a valid interrogation. I

The second type of reflected interrogation that may cause trouble follows a path to a transponder that is outside of the interrogator antennas main beam. These reflections persist for essentially the same length of time as the main-beam reflected interrogations. They also produce ghosts, but their ghosts are in line with the reflecting object rather than with any transponder.

The transponders now in use throughout the United States and Europe reduce the effect of this second type of reflected interrogations by suppressing all transponder interrogation decoders for about 35 sec (35 i 10) immediately after receiving a sidelobe interrogation signal so that no response can be given to a sidelobe interrogation. Recently, the Federal Aviation Adminstration has started equipping its interrogators with an Improved Interrogation Side Lobe Suppression (ISLS) technique that will cause all transponders within range but not in the main beam to be suppressed for about 35 psec each time an interrogation is transmitted. This is accomplished by transmitting two pulses omni-directionally, instead of the single pulse now transmitted for ISLS. The prime purpose of this change is to suppress reflected interrogations that reach transponders located outside of both interrogator antenna main beams and sidelobes. Even though the ISLS periods are only about 35 usec long, analyses have shown that they can be a serious reliability-reducing factor in those environments where large numbers of interrogators are operating. The reliability reduction could become particularly serious if large numbers of interrogator sites use the lmproved" lSLS now advocated by the Federal Aviation Administration unless firm controls over transmitter power are exercised. There is also a good possibility that the 35 user: suppression time now being used will not be long enough to prevent responses to non-main-beam reflected interrogations when air traffic becomes more dense. But this interval cannot be lengthened in present transponders without discriminating severely against other valid interrogations.

Reflected interrogations that reach transponders outside of the interrogator antennas main beam can be rejected by a technique quite similar to that described in aforementioned Patent Application Ser. No. 22,467 for rejecting reflected interrogations that reach transponders in the main beam. As described in Patent Application Ser. No. 58,321, filed July 27, 1970, in behalf of Walton B. Bishop, the time between arrival of the sidelobe suppression signal and arrival of the first reflected interrogation can be measured, and then this time can be used to reject succeeding reflected interrogations from the same source until the geometry of reflection changes.

Prior art transponders, such as disclosed by Majerus et al in U.S. Pat. No. 3,176,291 utilized a long suppression pulse upon receiving the sidelobe suppression signal. Clock, in U.S. Pat. No. 3,178,706, uses a suppression signal on the order of 30 microseconds upon receiving the sidelobe signal. There is no need to suppress the actual decoding of sidelobe interrogations in transponders as long as every successful decode is followed by a decoder suppression time that is considerably longer than the interrogation. There is a distinct advantage, however, in preventing unwanted interrogations from producing reply triggers if circuitry to reject reflected interrogations is to be used and decoder suppression times are shortened as they should be.

With a suitable technique disclosed for the prevention of reflected lFF interrogations triggering a transponder outside the main beam of the interrogating antenna without suppressing the transponder for 35 usec, a system for preventing the transponder from responding to the sidelobe interrogation without disabling the decoder for valid interrogations proves very desirable. The suppression of sidelobe interrogations and reflected interrogations has proved very desirable. However, main-beam direct-path interrogations from other interrogators are also suppressed during this interval, and these suppressions are not desirable. They reduce the capacity and reliability of the radar beacon system.

The suppression of interrogator antenna sidelobes is essential for the operation of current radar beacon systems in order to provide adequate trafflc capacity and azimuth discrimination. Also, the number of false targets continue to increase as the number of users increases and more buildings are constructed near interrogation sites. Consequently, a means of enabling transponders to reply to valid interrogations without affecting the current sidelobe suppression capability is urgently needed.

It has long been recognized that the use of long suppression periods to overcome reflections is wasteful, since it denies utilization of the transponder to other direct path interrogations that may arrive from other interrogators during the suppression period. In U.S. Pat. Application Ser. No. 59,972, filed July 3 l 1970, in behalf of Carlyle V. Parker and Walton B. Bishop, a technique utilizing delay lines or shift registers was disclosed to prevent an [FF transponder from responding to sidelobe interrogations without having a deadtime" of 35 psec.

With circuitry disclosed for a main-beam reflected interrogation rejector, a nonmain-beam reflected interrogation rejector, and a sidelobe interrogation decode suppressor, a need arose to interconnect these three distinct apparatus with a transponder to provide omnidirectional reflected interrogation rejection with instantaneous sidelobe interrogation decode suppression.

SUMMARY OF THE INVENTION The general purpose of this invention is to provide an improved suppression network to be used in an [FF transponder to prevent it from responding to reflected interrogation of all types main beam and non main beam, and to sidelobe interrogations without turning the transponder off and thereby preventing it from responding to valid interrogations. Logic circuitry is disclosed on how to interconnect the three distinct sets of logic circuitry previously disclosed without affecting the manner in which responses are made to main beam interrogations.

OBJECTS OF THE INVENTION It is therefore an object of the present invention to provide a transponder having an improved suppression network.

Another object of the present invention is to provide a transponder capable of recognizing reflected signals.

Another object of the present invention is to provide a transponder capable of recognizing sidelobe suppression signals.

A further object of the present invention is to provide a transponder operable without long suppression periods.

A still further object of the present invention is to provide an lFF transponder capable of responding to more valid interrogations.

Yet another object of the present invention is to increase the traffic capacity and reliability of current lFF systems.

A still further object of the present invention is to provide a way to prevent sidelobe interrogations from eliciting responses from transponders while allowing them to respond to valid direct interrogations at the same time.

Another object of the present invention is to provide a transponder with omnidirectional reflected interrogation rejection.

Yet another object of the present invention is to provide a transponder with the capabilities of omnidirectional reflected interrogation rejection and instantaneous sidelobe interrogation suppression.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

BRIEF DESCRlPTlON OF THE DRAWINGS FIG. 1 is a pictorial representation in which way lFF interrogations produce undesired transponder triggering;

FIG. 2 is a time plot of interrogation and reply signals in a transponder;

FIG. 3 is a block diagram of the rejection network for reflected main beam IFF interrogations;

FIG. 4a through FIG. 4g are a series of waveforms of the interrogation sidelobe suppression signal in conjunction with the interrogation pulse for the different modes currently in use in the IFF Mark X (SIF) System;

FIG. 5 is a block diagram of the rejection network for reflected IFF interrogations not in the main beam of the interrogating antenna;

FIG. 6 is one embodiment of the sidelobe interrogation decode suppressor;

FIG. 7 is a block diagram of the omnidirectional reflected interrogation rejector;

FIG. 8 is a block diagram of the omnidirectional reflected interrogation rejector combined with sidelobe interrogation decode suppressor;

FIG. 9 shows typical sequences of IFF interrogations and the transponder replies elicited when using the techniques disclosed in the invention, and

FIG. 10 is a time plot showing how use of new techniques reduces transponder dead time.

DESCRIPTION OF THE PREFERRED EMBODIMENT There are two types of reflected interrogations that interfere with the operation of radar beacons. The most common type follows a path that is entirely within the main beam of the interrogator antenna. The mainbeam reflection shown in FIG. 1 is of this type. When the geometry of interrogator-responsor, reflector, and transponder becomes as shown in the main beam of FIG. 1, not just one, but many, reflected interrogations reach the transponder. In fact, such reflected interrogations will continue to reach the transponder until the interrogator antennas main beam has moved off of the target (or the target has moved out of the main beam). Normally, as long as current equipment and interrogation repetition frequencies are used, each time a reflected interrogation reaches a transponder, about 30 to 40 more follow. Efforts to reduce interrogation repetition frequencies should eventually reduce this number to something less than twenty, but this will not change the reflected-interrogation problem. When main-beam reflected interrogations occur as shown in FIG. 1, a false target, or ghost, as shown at position A will appear in line with the real target and at a greater range. The range difference between real and false target will be equal to the difference between the lengths of the direct-path and the indirect-or reflected path between interrogator and transponder.

Transponders now in use throughout the United States and Europe reduce the effect of reflected interrogations in the main beam by suppressing all transponder interrogations for about 125 p.sec immediately after receipt of a main-beam interrogation. This suppression serves to deny a response to any interrogation arriving via a reflected path, but it also denies responses to other direct-path interrogations that may arrive from other interrogators during this time interval.

FIG. 2 shows how interrogations and reply triggers will appear at a transponder where the reflected-path interrogations are subjected to a 36-microsecond path delay. Reply trigger No. 1 is due to the arrival of a direct path interrogation, and N0. 2 is due to the arrival of its reflected path counterpart. The delay between triggers No. 1 and No. 2 can be measured and numbered in microseconds (36 in the case illustrated). Succeeding reflected interrogations that occur the same length of time after direct-path interrogations can then be rejected by preventing their reply triggers from reaching the transponders transmitter. (e.g., reply triggers No.4and No.6in FIG. 2).

A technique for preventing such reflected IFF interrogations from triggering an aircraft transponder without having this long suppression period is taught and claimed in copending patent application of Walton B. Bishop, Ser. No. 22,467, filed Mar. 25, 1970. That patent application describes a technique for overcoming the effect of IFF interrogation reflections whose entire path is within the main beam of the interrogator antenna. The technique consists of measuring the time between arrival of the direct-path and the first indirectpath interrogations, and then using this information to reject succeeding indirect-path interrogations of the same delay. It thus eliminates the need for the long usec) transponder decoder suppression periods that follow reception of a valid interrogation.

FIG. 3 shows a simple circuit that can accomplish the reflected IFF interrogation rejection illustrated in FIG. 2.

The counter 14 is normally in the all-zero setting; the switch 15 is thus normally in the reset position so that there is no output on lead Q. Hence, although the 1 MHz pulse generator 18 is operating continuously, none of its pulses can pass through AND circuit 19 until switch 15 is set. AND circuit 21 is so connected that it will cause switch 15 to be reset each time the counter 14 reaches the all-zero state. The radix converter accepts a binary number from the counter 14 each time the read bus 23 is activated and produces a pulse on the leads 1 to 127 in the memory circuit 24 that corresponds to this binary number. Note that the memory circuit 24 remembers which leads have been activated during a period equal to the average scan time, i.e., the average time that an interrogator antenna is beamed toward a transponder.

The operation of the circuit shown in FIG. 3 will now be described in reference to FIG. 2 signals appearing at IN terminal 25. Reply trigger No. 1 will immediately cause the state of the counter 14 to be read into the radix converter 22. However, since the counter 14 is normally in the all-zero state, and there is no 0" terminal at the output of the radix converter 22, reading" the counter at this time will produce no results. Reply trigger No. 1 will be delayed very slightly by delay 26 and then it will set switch 15, thus producing a continuous output on lead 0 and providing one input to AND gate 19. Pulses from the continuously running 1 MHz pulse generator 18 will then start entering the seven-stage binary counter 14 at the rate of one pulse per microsecond. The 1 MHz rate is used for illustration only as other rates could be utilized just as well. Since there is no output from the OR circuit 27, reply trigger No. 1 will pass directly through delay 28 and AND circuit 29 to the OUT terminal 30. The delay 28 must only be long enough to permit sensing the status of the rejector circuit. It should notbe greater than 0.5 microsecond. The other delays, 26 and 31, should be still smaller than delay 28.

Reply trigger No. 2, the first'trigger resulting from a reflected-path interrogation, occurs 36 microseconds after reply trigger No. 1. It will thus find the counter 14 on binary 0100100. This number, when read through the radix converter 24 will receive a pulse. This pulse will enter the D-circuit of lead No. 36 (See typical example shown on lead No. 127), and after a slight delay caused by delay 31, it will activate the monostable switch 32. Switch 32 will remain activated and thus provide one input to AND circuit 33 for the duration of an average scan. However, there will be no input to OR circuit 27 until a second pulse enters the same D-circuit. Long before reply trigger No. 2 (of FIG. 2) occurs, the counter 14 will have reached the all-zero state and hence will have caused switch to be reset. Reply trigger No. 3 will thus accomplish exactly the same thing as reply trigger No. l it will allow the counter 14 to start counting pulses from the pulse generator 18, and it will produce a reply trigger on the OUT terminal 30.

When reply trigger No. 4 arrives (36 microseconds after reply trigger No. 3) it will find the counter 14 in state OIOOIOO just as reply trigger No. 2 did; so lead No. 36 to the memory circuit 24 will again receive a pulse. This pulse will pass directly through the D-circuit of lead No. 36, for monostable switch 32 will still be providing one input to AND 33. The monostable switch 34 serves to lengthen the pulse that comes out of the D- circuit 16 so that, after passing through the OR circuit 27, it will inhibit AND circuit 29 and thus prevent reply trigger No. 4 from producing an output at the OUT terminal 30.

In like manner, reply trigger No. 5 will produce a reply trigger at the OUT terminal 30 and reply trigger No. 6 will not, since these triggers have exactly the same time relationship as reply triggers No. 3 and No. 4. All succeeding pairs of reply triggers separated by 36 microseconds will do the same, i.e., the reply trigger resulting from a direct-path interrogation will produce a reply trigger on the OUT terminal 30 and the one resulting from a reflected-path interrogation will not.

Only one reflected IFF interrogation rejector circuit is needed in a transponder, for the operations described above can be performed simultaneously for other pairs of reply triggers separated by any length of time up to the capacity of the counter 14 (127 microseconds in the example illustrated).

In current transponders, the first 21 leads and the corresponding D-circuits may be omitted, because all transponders are automatically suppressed while the reply of approximately 2l microseconds is being transmitted. Whether the leads numbered 91 through 127 are needed or not is open to question, since leads 22 through 90 would provide anti-reflection protection equal to that which is now being provided.

The second type of reflected interrogations that may cause trouble follows a path to a transponder that is outside of the interrogator antennas main beam. The non-main-beam reflection shown in FIG. 1 is of this type. These reflections persist for essentially the same length of time as the main-beam reflected interrogations. They also produce ghosts, but their ghosts are in line with the reflecting object rather than with any transponder. The ghost labeled B in FIG. 1 is typical.

The transponders now in use throughout the United States and Europe reduce the effect of reflected interrogations by suppressing all transponder interrogation decoders for about 35 usec (35 i 10) immediately after receiving a sidelobe interrogation. Recently, the Federal Aviation Administration has started equipping its interrogators with an Improved" interrogation side lobe suppression (ISLS) technique that will cause all transponders within range but not in the main beam to be suppressed for about 35 psec each time an interrogation is transmitted. This is accomplished by transmitting two pulses omnidirectionally, instead of the single pulse now transmitted for ISLS. The prime purpose of this change is to suppress reflected interrogations that reach transponders located outside of both interrogator antenna main beams and sidelobes.

Radar beacon interrogators currently in use in the United States and Europe for air traffic control use are the Air Traffic Control Radar Beacon System or IFF Mark X (SIF). The Mark X (SIF) has several different modes, some for use in civilian air traffic control and some in exclusive military use. It should be noted that the techniques to be described in this patent application can be applied with only very slight modification to the classified modes of the IFF Mark XII.

FIGS. 4a through 4g illustrate the sidelobe suppression signal P, and P followed by the interrogation pulse P The sidelobe suppression signal consists of a pair of pulses separated by two microseconds as shown in FIG. 4a. For those transponders in the main beam of the interrogating antenna, the P pulse is not detected and only the P pulse followed by the interrogating pulse P;, is detected. For those transponders outside the main beam of the interrogating antenna, the current practice is to suppress the transponder decoder upon reception of the two pulses (P P separated by 2 microseconds for a period of 35 microseconds. During this interval, the transponder is unable to decode any interrogations, including those arriving in the main beam of another interrogator (without the P pulse present). FIGS. 4b through 4g illustrate the spacing between pulses in the different modes of the IFF Mark X (SIF') system. For example, the P pulse follows the P, pulse by 3 microseconds in the Mode 1 configuration, by 5 microseconds in the Mode 2 configuration, by 8 microseconds in the Mode 3/A configuration, by 17 microseconds in the Mode B configuration, by 21 microseconds in the Mode C configuration, by 25 microseconds in the Mode D configuration.

Reflected interrogations that reach transponders outside of the interrogator antennas main beam can be rejected by a technique quite similar to that described in US Pat. Application Ser. No. 22,467 for rejecting reflected interrogations that reach transponders in the main beam. As disclosed in US. Pat. Application Ser. No. 58,321, filed July 27, 1970, on behalf ofWalton B. Bishop, the time between arrival of a two-pulse sidelobe suppression signal and arrival of the first reflected interrogation can be measured, and then this time can be used to reject succeeding reflected interrogations from the same source until the geometry of reflection changes.

Referring now to FIG. 5, there is shown the non-main beam reflected-interrogation rejector. As evident, this circuit has no effect upon operation of the transponder 115 when main-beam interrogations (without ISLS pulse pairs) are received. The reply triggers from the transponder receiver 111 pass through AND gate 112 and OR gate 113 to the transmitter 114.

When a pair of pulses spaced 2 1sec apart (an ISLS pair as shown in FIG. 4 are received, however, they will produce two outputs from stages and 2 of the shift register 116 which shifts at a 1 MHz and hence AND gate 117 will be enabled and produce a pulse. This pulse does three things: (1) It sets the counter 118 to zero if the counter is not already in that state. (2) After a very short delay produced by delay 119 it sets switch 120 which causes an output on lead Q, thus allowing pulses from the continuously-running pulse generator 121 to pass through AND gate 122 to the counter 118. Pulse generator 121 operates at the lMHz rate for simplicity, although other rates could be used as well. The -stage digital counter 118 will count up to 32 and then stop automatically at the all-zero state. (3) The pulse from AND gate 117 will also cause monostable switch 123 to generate a gate equal to or slightly larger than the time required for the counter to complete a full cycle (32 sec in this example).

The 32 sec gate from monostable switch 123 will inhibit AND gate 112 so that no reply triggers can pass through it and simultaneously it will provide one input to AND gate 124 so that reply triggers will pass through it. If a reflected interrogation arrives during the 32 usec gate, the reply trigger it produces will read the binary number from the counter 118 into the radix converter 125 and thus cause a pulse to enter the lead in the memory circuit 126 that corresponds to this number. The first pulse into a D-circuit in memory circuit 126 will cause monostable switch 127 (after a slight delay in delay 132) to produce a gate which provides one input to AND gate 128 so that the next pulse will produce an output to OR gate 135. The gate from monostable switch 127 lasts for a length of time slightly greater than the time it takes an interogator antenna to scan past a reflecting object, essentially the same length of time it takes the antenna to scan past a responding target. However, there will be no output from AND gate 128 until a second pulse enters the same D-circuit. A second, third, fourth, etc. pulse will enter this samd D- circuit, if-and-only-if, succeeding ISLS pulse pairs are followed by reflected interrogations having the same delay as the one which activated monostable switch 127.

Long before an ensuing ISLS pulse pair arrives, counter 118 will reach the all-zero state and reset switch 120 through AND gate 133. Upon reception of the next ISLS pulse pair, switch 120 will cause counter 118 to start counting again, and if a reflected main beam signal arrives at the transponder at the same spacing from the ISLS pulse pair as the first reflected interrogation, it will find the counter 118 in the same state thus the same number read by radix converter 125 and the same lead corresponding to this number in memory circuit 126 enabled, and thus AND gate 128 is enabled. Each time a reflected interrogation produces an output from AND gate 128, monostable switch 129 produces a short gate that acts through OR gate 135 to inhibit AND gate 130. Reply triggers from all but the first reflected interrogation from any object are thus rejected. Note that delay 131 serves only to delay the reply triggers enough to allow the inhibit gate to be effective on gate 130.

FIG. 5 includes a special feature, considered unnecessary for main-beam reflections, that will cause the gate from monostable switch 123 to be regenerated any time that a second ISLS pulse pair is received before the 32 psec interval following the first ISLS pair has elapsed. When such an ISLS pulse pair is received, the pulse from AND gate 117 resets the counter 118 via lead 134 and thus will allow a second reply trigger from reflected interrogations in the process of being rejected to pass through AND gate 130. The regenerated gate from monostable switch 123 however will then continue to reject reflected interrogations for a full 32 psec.

The counter 118 of FIG. 5 shows only 5 stages so that it will provide reflected-interrogation rejection equivalent to that now required by the specification which calls for a 35 i 10 p.860 suppression gate following reception of an ISLS pulse pair. If a longer gate is required to reject main-beam reflected interrogations, a longer gate should be used to reject non-main beam ones. A six stage counter corresponding to a 64 p.566 gate may be used. If the counter 118 is only capable of counting to 32 it is not possible for two reflections of the same interrogation to trigger a response. If the counter is made to count to as high as 64, then the circuit of FIG. 5 can handle multiple reflections of an interrogation.

The technique described above can be used whether the sidelobe suppression signals are accompanied by direct-path interrogations or not when the Improved ISLS technique is used, i.e., when the pulse pairs are transmitted omnidirectionally. Hence, all but the first non-main beam reflected interrogation that could at present produce a false target can be rejected in all regions where either a sidelobe interrogation and a standard ISLS pulse are both received or Improved ISLS signals alone can be detected.

In addition to reflections causing unwanted triggering of the transponder, sidelobe interrogations can cause the transponder to reply to the interrogations. As explained above, current practice is to suppress the transponder for a full 35 psec upon reception of the sidelobe suppression signal P P In order to allow a transponder to have the ability to decode main beam interrogations from other interrogators rather than disable it for a full 35 microseconds, FIG. 6 shows an embodiment on how to allow the decoder to decode valid interrogations, and yet be inhibited upon detection of a sidelobe suppression signal. This embodiment is one of three disclosed in U.S. Pat. Application Ser. No. 59,972, filed July 31, l970, on behalf of Carlyle V. Parker and Walton 13. Bishop. More specifically, FIG. 6 shows the video pulses in the transponder being fed to a shift register 211 having 26 stages, shifting at a 1 MHz rate. A sidelobe suppression signal, consisting. of two pulses spaced by 2 microseconds, would result in a binary l in stages 0 and 2 of the shift register 211, thus enabling AND gate 212. The output of AND gate 212 is fed back to stages 0 and 2 of the shift register 211, thus enabling AND gate 212. The output of AND gate 212 is fed back to stages 0 and 2 of shift register 211 on lead 219 in such a manner that it changes the binary l in each of these stages to a binary before the next shift occurs. This is equivalent to removing P, and P from all SIF (Mark X) codes and thus none of the sidelobe suppression signals will produce a reply trigger, while main beam interrogations will produce a reply trigger on their appropriate leads. For example, in Mode 3/A, if a sidelobe suppression signal (P, and P,,) were present, AND gate 212 would effectively remove P, and P, from stages 0 and 2 before they were shifted into stages 1 and 3 respectively. Thus when the P, pulse arrived in stage 0 6 microseconds later, AND gate 215, having its inputs from stages 0 and 8, would not be enabled since a binary 0 would be found in stage 8. If, however, the interrogation were a main beam one, the P pulse would not be present in stage 0 when the P, pulse was in stage 2 and thus AND gate 212 would not be enabled. The binary 1 in stage 2 would be shifted down to stage 8 when the P, pulse would be in stage 0, thus enabling AND gate 215 and eliciting a response to the interrogation. AND gates 213, 214, 216, 217 and 218 would be the equivalents to AND gate 215 for Modes 1, 2, B, C, and D, respectively.

The main-beam reflected interrogation rejector (MABERIlR) shown in FIG. 3 and the non-main-beam reflected-interrogation rejector (NONMABERIR) shown in FIG. 5 may be combined in at least two ways to provide an Omnidirectional reflected-interrogation rejector (OMNIRIR). FIG. 7 shows how these two circuits may be combined and made to work with only a slight modification to existing equipment. The present transponder decoder suppression circuitry must be disabled and reply triggers for all modes must be fed into the MABERIR 311, and the undecoded video pulses must be fed into the NONMABERIR 312.

When the circuits are interconnected as shown in FIG. 7, the MABERIR will inhibit the reply triggers for all but the first main-beam reflected interrogation of each delay. The NONMABERIR will then simply pass the uninhibited reply triggers directly on to the transponder transmitter. Of course, the current method of determining which mode of interrogation should be associated with each reply trigger must be maintained.

The above process is reversed when non-main-beam reflected interrogations are received. The MABERIR. 311 will pass reply triggers resulting from non-mainbeam interrogations directly to the NONMABERIR 312 which will inhibit the reply triggers for all but the first non-main-beam reflected interrogation of each delay.

The OMNIRIR of FIG. 7 is advantageous in that it requires fewer modifications to existing equipment than the one shown in FIG. 8. However, the circuit of FIG. 8 would be considerably more effective in increasing transponder traffic capacity and/or reliability.

Referring now to FIG. 8, the sidelobe-interrogation decode suppressor (SIDES) 413, main-beam-retlectedinterrogation rejector (MABERIR) 411 and non-mainbeam-reflected-interrogation rejector (NON- MABERIR) 412 are interconnected with a transponder 414 to provide omnidirectional reflected-interrogation rejection and instantaneous sidelobe-interrogation decode suppression. Other embodiments disclosed in U.S. Pat. Application Ser. No. 59,972 could be substituted for SIDES 413 just as well. AND circuits 415 through 421 with the special feedback circuit to stages 0 and 2 of the shift register 422 described earlier with respect to FIG. 6, will prevent the decoding of all direct-path sidelobe interrogations, and will decode all other interrogations. The reply triggers for these interrogations will be delayed a very small fraction of a microsecond by the delay circuits 423 through 428 so that appropriate reply-trigger gates can be generated by monostable switch 429 when the triggers are not due to reflected interrogations that should, and can, be rejected. All reply triggers also pass through OR gate 430 to the MABERIR 41 1, the details of which are shown in FIG. 3. Main-beam direct-path interrogations and the first indirect-path main-beam interrogation will produce reply triggers at the output of this circuit. Such triggers will pass directly through the NONMABERIR 412 of FIG. 5 to monostable 429 which will produce a gate just long enough to allow the slightly-delayed trigger to pass through AND gates 431 through 436 to the transponder transmitter 437.

When non-main-beam reflected interrogations are decoded, their reply triggers will pass directly through the MABERIR 411, but only the first from each reflecting object will pass through the NONMABERIR 412.

FIG. 9 shows some typical sequences of interrogations and sidelobe suppression pulse pairs that might be received by a transponder, and the reply triggers that they will generate when the OMNIRIRS of FIGS. 7 and 8 are used. Two sequences (i.e., multiple reflections of each interrogation) of main-beam reflected-interrogations and one sequence of non-main-beam ones are shown. The three sets of signals received are assumed to occur separately. The period 8, represents the time during which main-beam reflected interrogations will be rejected (127 usec in the illustration of FIG. 3), while 8 represents the time during which non-mainbeam reflected interrogations will be rejected (32 psec in the illustration of FIG. 5).

The manner in which reception of two or more sidelobe suppression pulse pairs in a single 6, interval serves to lengthen 8, is not shown in FIG. 9, but its importance should not be overlooked. This feature (due to the automatic resetting of the counter shown in FIG. 5) not only permits the NONMABERIR to reject reflected interrogations from a number of interrogators simultaneously, but it also causes 8, to be lengthened any time a sidelobe interrogation pulse-pair arrives by an indirect path.

As mentioned earlier, the periods 6, and 8, should probably be the same length. There may be some interrogator sites, particularly those surrounded by many large buildings, that would benefit by having 8 greater than 8,. However, since the reflected interrogation re jectors are for use with transponders, and no restrictions can be placed upon a transponders movements, values for 8, and 8, must be chosen so that they will be satisfactory for all environments. The geometry of FIG. 1 shows clearly that a non-main beam reflected inter rogation can follow a longer path and hence be delayed more than a main-beam reflected interrogation. However, it is quite likely that many reflected interrogations, especially those that follow the longest paths, will be too weak to trigger a response from a tranpsonder and hence will not require a reflected-interrogation rejector.

FIG. shows how use of new techniques (the MABERIR, the NONMABERIR, or the OMNIRIR) reduces transponder dead time, the time that a transponder is suppressed, after having received an interrogation or an ISLS pulse pair. The dead time of approximately 25 usec, shown when the new techniques are used, represents only the time required for the transponder to generate a reply. No other suppression is needed. Only three interrogation repetition periods are shown for each of thefour cases illustrated. Succeeding interrogations, reflections and sidelobe sup- 7 pression signals would produce dead time exactly like those shown for the second and third until the memory circuits become deactivated.

These circuits may easily be adjusted to remain activated for the maximum length of time that an interrogator interrogates a given transponder as the interrogator antenna scans past the transponder. Interrogators with non-scanning antennas, which may remain on a given target for long periods, must interrogate intermittently, and if full capacity of the system is to be realized, they should interrogate in bursts, each lasting approximately the same length of time as the present duration of a scan past a target.

The manner in which multiple reflections of the same interrogation are rejected requires some further explanation. The first indirect-path interrogation to arrive will trigger a response from the transponders transmitter, thus suppressing its receiver. If a second indirect-path interrogation arrives before the transponders receiver is enabled again (about25 usec) the receiver will be unable to detect the second indirectpath interrogation. This means that when a sequence of interrogations arrive by a direct path and by two indirect paths, the first shorter-path reflected interrogation will elicit a response, but none of the succeeding shorter-path ones will, while the longer-path reflected interrogations will not elicit a response until the second one arrives. Of course, all succeeding longer path reflected interrogations will be rejected. The process may continue for more than two multiple-reflections of an interrogation. In all cases, however, no more than one reflected interrogation of a given path length will trigger a response. Even if all of the reflected interrogations were rejected, it would still be possible for replies from direct-path interrogations to return by two or more paths that are entirely in the main beam and thus produce false targets. Special circuitry in the interrogator-responsor should be able to discriminate against any false targets produced in this manner. In fact, proper setting of the gain-time-control circuit should adjust the receiver sensitivity so that only the direct path replies would produce outputs to the display. Use can also be made of the information in coded replies to recognize and reject the reflected replies if amplitude discrimination is not adequate.

An omnidirectional reflected-interrogation rejector (OMNIRIR) of the type shown in either FIG. 7 or FIG. 8 is capable of causing a transponder to reject all but one of the interrogations reflected from any object to it. This reduction in responses to reflected interrogations is believed to be sufficient to warrant eliminating all transponder dead time beyond that required for generating a reply. FIG. 8 includes a simple side-lobeinterrogation decode suppressor (SIDES) circuit which prevents responses to direct-path sidelobe interrogations without suppressing the transponder and thus provides an important increase in capacity and/or reliability of the radar-beacon system. The circuit of FIG. 7, providing the same reflected-interrogation rejection capability as FIG. 8 but without the instantaneous sidelobe interrogation decode suppression (SIDES) circuit, could be implemented with fewer modifications to existing equipment.

It should be noted that the reflected-interrogation rejectors described in FIGS. 3 and 5 will also reject all but one of the valid interrogations from all but one of two or more interrogators operating at interrogation-repetition periods that differ by less than one microsecond any time that such interrogators interrogate a particular transponder simultaneously. Interrogation-repetition frequency assignments usually make this situation extremely unlikely, or even impossible. It should also be noted that the techniques described in this report can be applied with only very slight modifications to the classified Mode 4 of the IFF Mark XII.

The use of this reflected IFF interrogation rejector would permit transponders to answer many valid interrogations that now fail to be answered because of he long suppression periods. This would serve to increase the number of interrogators that could use the IFF system simultaneously without experiencing mutual interference and would automatically increase the systems reliability and anti-jam capability. The fact that transponder transmitter duty cycles might be increased momentarily should not be a problem, since the automatic overload control (AOC) circuit protectsthe transmitters from overloading. Very few valid interrogations would be rejected by the one microsecond suppression intervals, associated with reflected interrogations, in the memory circuits.

It is quite likely that a number of simplifications are feasible. It might be possible, for example, to reduce the counting rate of the counter and thus reduce the number of D-circuits required. The suppression signal need not be the second of three pulses but may be a pulse at the end of an interrogation, and the circuitry could be modified accordingly. Good systems engineering and circuit design should provide a number of other simplifications. It is believed that the approach of suppressing only the reflections and sidelobe interrogations that actually reach a transponder offers the possibility of considerable improvement in transponder operation. The same approach should be applicable to certain other electronic systems that provide navigation and/or communications information.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. In a transponder receiving IFF interrogation signals and emitting reply signals, a suppression network for suppressing reply signals to reflected interrogation and sidelobe interrogation comprising: I

first means for determining the presence of a sidelobe suppression signal, said first means including delay means for delaying said interrogation signals, first coincidence means for detecting the presence of two pulses spaced 2 microseconds apart in said delaying means, second coincidence means for passing a pulse upon the nonpresence of said two pulse signal;

second means for determining if said interrogation is a reflected main beam interrogation;

third means for determining if said interrogation is a reflected non-main beam interrogation;

an OR gate connected between said second coincidence and said second determining means;

whereby said transponder is suppressed from emitting a reply signal if any of said determining means indicates the presence of a sidelobe or reflected interrogation.

2. A suppression network as recited in claim 1, in-

cluding:

third coincidence means connected to said second coincidence means and said third determining means whereby said third coincidence means output signal triggers said transponder transmitter and wherein said third determining means supplies a pulse to said third coincidence means only upon determining that said interrogation is not a reflected non-beam interrogation and that said second determining means has supplied an indicating signal that said interrogation is not a reflected main beam interrogation.

3. A suppression network as recited in claim 1 wherein said second determining means comprises:

means for determining the time interval between a second interrogation following a first interrogation within a certain period of time;

means for storing said determined time interval;

means for indicating to said third determining means that no two stored time intervals are identical.

4. A suppression network as recited in claim 3 wherein said third determining means comprises: 

1. In a transponder receiving IFF interrogation signals and emitting reply signals, a suppression network for suppressing reply signals to reflected interrogation and sidelobe interrogation comprising: first means for determining the presence of a sidelobe suppression signal, said first means including delay means for delaying said interrogation signals, first coincidence means for detecting the presence of two pulses spaced 2 microseconds apart in said delaying means, second coincidence means for passing a pulse upon the nonpresence of said two pulse signal; second means for determining if said interrogation is a reflected main beam interrogation; third means for determining if said interrogation is a reflected non-main beam interrogation; an OR gate connected between said second coincidence and said second determining means; whereby said transponder is suppressed from emitting a reply signal if any of said determining means indicates the presence of a sidelobe or reflected interrogation.
 1. In a transponder receiving IFF interrogation signals and emitting reply signals, a suppression network for suppressing reply signals to reflected interrogation and sidelobe interrogation comprising: first means for determining the presence of a sidelobe suppression signal, said first means including delay means for delaying said interrogation signals, first coincidence means for detecting the presence of two pulses spaced 2 microseconds apart in said delaying means, second coincidence means for passing a pulse upon the nonpresence of said two pulse signal; second means for determining if said interrogation is a reflected main beam interrogation; third means for determining if said interrogation is a reflected non-main beam interrogation; an OR gate connected between said second coincidence and said second determining means; whereby said transponder is suppressed from emitting a reply signal if any of said determining means indicates the presence of a sidelobe or reflected interrogation.
 2. A suppression network as recited in claim 1, including: third coincidence means connected to said second coincidence means and said third determining means whereby said third coincidence means output signal triggers said transponder transmitter and wherein said third determining means supplies a pulse to said third coincidence means only upon determining that said interrogation is not a reflected non-beam interrogation and that said second determining means has supplied an indicating signal that said interrogation is not a reflected main beam interrogation.
 3. A suppression network as recited in claim 1 wherein said second determining means comprises: means for determining the time interval between a second interrogation following a first interrogation within a certain period of time; means for storing said determined time interval; means for indicating to said third determining means that no two stored time intervals are identical. 